Tuberculosis is the leading cause of death from infectious diseases worldwide, with increasing consequences due to the HIV epidemic. The existing vaccine, BCG, has marginal efficacy. Infection and disease are caused by the intracellular bacterial pathogen Mycobacterium tuberculosis. In the face of a complex immune response, the bacteria can persist indefinitely in granulomas, tight aggregates of highly differentiated macrophages and other immune cells. As with most infectious diseases, the impact of the different aspects of host immunity on tuberculous infection and disease are not well understood. To better understand the host responses to tuberculous infections, we will exploit the fact that Mycobacterium marinum, a close relative of M. tuberculosis is a natural pathogen of zebrafish, causing a tuberculosis-like disease. Zebrafish are genetically tractable vertebrates that are used as models of disease and development. We have exploited the optical transparency of the zebrafish embryo to visualize M. marinum infection of embryonic macrophages in real time. The infection parallels adult tuberculosis in many ways. We will use the zebrafish-M, marinum infection model to study the role of host immune genes in infection. We will use whole mount in situ hybridizations to determine which host immune markers thought to be important in tuberculosis, are expressed in the developing embryo. We will determine if their expression is changed during infection. We will inactivate these genes using morpholino technology and determine by real time visualization when and how they act in infection. We will determine their role for the different facets of infection, from early macrophage migration in response to M. marinum to granuloma formation. The ability to visualize infection in real time in zebrafish embryos with functional inactivation of individual host genes will allow us to study the role of the host immune system in unprecedented detail. The proposed experiments will inherently yield new information about the role of various immune genes in zebrafish development.